Ophthalmic analysis system for measuring the intraocular pressure in the eye

ABSTRACT

The invention relates to an ophthalmic analysis system ( 01 ) for measuring the intraocular pressure in an eye ( 02 ) comprising 
         a) an actuating device ( 04 ) for contact-free deformation of the cornea ( 03 ),    b) an observation system ( 20, 21 ) with which the deformation of the cornea can be observed and recorded,    c) an analysis device ( 24 ) with which the intraocular pressure can be deduced from the image information of the observation system ( 20, 21 ), wherein split images of at least parts of the undeformed and/or deformed cornea ( 03 ) can be recorded using the observation system ( 20, 21 ).

FIELD

The invention relates to an ophthalmic analysis system for measuring theintraocular pressure in the eye (eye internal pressure) according to thepreamble of claim 1.

BACKGROUND

Serious health impairments can be triggered by an elevated intraocularpressure. In particular, the optic nerve can be damaged by the elevatedintraocular pressure which causes so-called glaucoma with restrictionsof the visual field.

Three basic principles are known for checking the intraocular pressure,namely impression tonometry, applanation tonometry and noncontacttonometry. The impression tonometer measures the depth of theindentability of the cornea caused by a metal stamp loaded with a knownweight. For the same weight the indentability is inversely proportionalto the intraocular pressure, that is the indentability is greater, thelower is the intraocular pressure and conversely. A disadvantage withimpression tonometry is that the placement of the tonometer and theimpression of the metal stamp additionally increase the intraocularpressure so that the measured pressure does not correspond exactly tothe actual intraocular pressure. Furthermore, the placement of the stampon the cornea of the patient's eye is relatively stressful for thepatients.

Furthermore, so-called applanation tonometers are also known formeasuring the intraocular pressure, its measurement being based onapplication of the applanation principle. The applanation principlestarts from Ingbert's law which states that the pressure in a sphericalcontainer filled with liquid corresponds to the counterpressure whichflattens a certain surface of this sphere. The intraocular pressure canbe measured on the basis of this law in two different ways. According toa first alternative, a tonometer with constant weight can be used andthe flattened surface can be measured. According to an alternativemethod of measurement, the force required to flatten a known surface ofconstant size is used. A Perkins applanation tonometer is known, whichconsists of a plastic cylinder whose lower planar end is provided with agradation. A magnifying glass is located at the upper end. Afterinstilling a fluorescent liquid into the conjunctival sac, the diameterof the applanated corneal surface can be determined by optical readingoff on the gradation scale. In this case, the intraocular pressure isdetermined by means of a constant force.

In addition, an applanation tonometer operating on the principle of anapplanated surface of constant size is known. In this case, the corneais flattened using the quadrilateral base of a glass prism. Theintraocular pressure is measured by intensifying the pressure of theprism on the eye until the flattened circular region of the cornea is atthe same level as the four sides of the prism base. A disadvantage withapplanation tonometers again is that as a result of the deformation ofthe cornea by means of an actuating element, considerable stress isproduced for the patients.

So-called noncontact tonometers were developed to avoid this stressingproduced by contact with a deforming tool. In these noncontacttonometers actuating devices are provided for deforming the cornea withwhich the cornea is deformed free from contact. For this purpose, a puffof compressed air is produced for example and directed onto the cornea.In known noncontact tonometers air puffs are directed onto the eye inthe direction of the optic axis whereby the cornea is increasinglyflattened and finally indented. To measure the deformation of thecornea, an obliquely incident bundle of parallel light rays is directedonto the cornea and the light reflected by the cornea is measured as ameasurement signal. For this purpose, the reflected light can beintercepted by a light sensor, for example, where the light intensitymeasured by the light sensor varies as a function of the applanation ofthe cornea caused by the air flow.

A disadvantage in all known methods of measurement is that whenmeasuring the intraocular pressure, the counterpressure caused by theelastic deformation of the cornea is not taken into account. This isbecause the cornea itself is stretched over the vitreous body in thefashion of an elastic membrane so that during the measurement of theintraocular pressure a certain amount of force is required for itsdeformation which is included in the measurement results in a falsifyingmanner. This falsification is of a different magnitude in differentpatients since the properties of the cornea, especially its thicknessand elasticity, vary within certain limits.

SUMMARY

Starting from this prior art, it is thus the object of the presentinvention to propose a new ophthalmic analysis system for measuring theintraocular pressure which avoids the disadvantages of the previouslyknown prior art.

This object is solved by an analysis system according to the teaching ofclaim 1.

Advantageous embodiments of the invention are the subject matter of thedependent claims.

The analysis system according to the invention is based on the basicidea of recording split images of the cornea before and/or during and/orafter the deformation of the cornea, which show the state of the corneain a plane of intersection. These split images are analyzed in theanalysis device by means of suitable image processing methods andprovide additional information on the state of the cornea which can betaken into account when deducing the intraocular pressure.

The thickness of the cornea has a major influence on the measured valuesand thus on the result of measurement of the intraocular pressure sincethe cornea as an elastically deformable membrane opposes the deformationforce applied by the actuating device with a counterforce which does notdepend on the intraocular pressure itself and can therefore falsify themeasurement of the intraocular pressure. It is thus particularlyadvantageous if the thickness of the cornea is deduced from split imagesof the cornea. Taking into account the known elasticity characteristicsof the cornea, the counterforce applied by the cornea during the elasticdeformation can be estimated from the thickness of the cornea and takeninto account as an influential factor when deducing the intraocularpressure.

Alternatively or additionally to determining the thickness of the corneaas an influential factor, the curvature of the cornea can also bederived from the split images of the cornea. The curvature of the corneaalso influences the measurement results and should thus be taken intoaccount when deducing the intraocular pressure.

In addition, it is also possible to determine the light scattering ofthe cornea from the split images of the cornea. The light scattering ofthe cornea has a specific relationship to the elasticity characteristicof the cornea so that the elasticity of the cornea can be deduced fromthe light scattering.

It is fundamentally arbitrary which method of measurement is used tomeasure the intraocular pressure itself and for the derivation in theanalysis system. For example, the known reflected light methods can beused for this purpose, where the split images recorded according to theinvention are then used, for example, merely to correct for theinfluence produced by the elastic deformation of the cornea. However, itis especially advantageous if the intraocular pressure is also deducedfrom the split image recordings of the deformed cornea. These splitimage recordings represent the deformation of the cornea caused by theactuating device extraordinarily exactly and thus contain the imageinformation required to deduce the intraocular pressure. For example,during the deformation of the cornea a plurality of split images can berecorded successively as a series of images so that the split image withthe greatest deformation of the cornea can then be extracted in thefollowing image analysis. The intraocular pressure can then simply bederived from this image of the cornea with the greatest deformationtaking into account the thickness of the cornea.

The equipment used to obtain the split image recordings of the cornea isfundamentally arbitrary. It is especially disadvantageous if theobservation system comprises a slit projector which can project a lightslit onto the cornea. Slit projectors of this type are known fromophthalmology. The necessary illumination principle of the slitprojector is based on the fact that the refractive media of the ocularanterior chamber are not transparent but significant scattering takesplace at said media, especially in the short-wavelength fraction of thevisible light. This has the result that a focused light beam, that is inthe present case the projected light slit which is passed through theoptical media of the eye, makes the ocular structures and in particularthe cornea visible as a split image when viewed laterally since thelight is scattered at different intensities on passage through thedifferent materials, especially on passage through the cornea. Theslit-shaped light beam thereby produces an image plane which runsthrough the ocular body in cross section so that the split images to berecorded using the observation system lie specifically in this imageplane defined by the light slit.

In order to be able to record the split images illuminated by the lightslit, the observation system should comprise a recording device which isarranged so that the image plane illuminated by the slit projector canbe recorded at least partly.

In order to enhance the image quality, at least one objective, that is alens arrangement can be arranged between cornea and recording device.Using this lens arrangement the image plane of the cornea illuminated bythe slit projector is imaged on a recording plane in the recordingdevice.

In order to achieve a large depth of focus in the split images, thearrangement of the image plane in the cornea illuminated by the slitprojector, the principal plane of the lens system between the cornea andthe recording device (objective plane) and the recording plane of therecording device should satisfy the Scheimpflug condition. This ruledeveloped from the photographs of Scheimpflug prescribes that the imageplane, the objective plane and the recording plane are arranged atangles such that they intersect in a common axis. By tilting therecording plane relative to the object plane, the image plane can bebrought into an arbitrary spatial position wherein image points aredetected in the depth of focus which cannot otherwise be sharply imagedat the same time when the image plane is perpendicular.

For contact-free deformation of the cornea it is particularlyadvantageous if a flow pulse of a gaseous medium, especially air, can beapplied to the surface of the cornea using the actuating device. Thestressing of the patients through such an air jet is relatively verysmall and is not perceived as very stressful because of its shortness.

The actuating device can be constructed by providing a pressure chamberwith a nozzle orifice directed onto the eye to be examined. As a resultof a short-term increase in the pressure in the pressure chamber, thegas located in the pressure chamber flows out through the nozzle orificeand in this way forms the desired flow pulse on the surface of the eye.The increase in the pressure in the pressure chamber can be achieved forexample, by moving a mechanically driven stamp in a cylindrical openingof the pressure chamber.

In order that the deformation of the cornea can be correlated with thestrength of the flow pulse, a sensor, for example, a pressure sensorshould be provided in or on the pressure chamber which can be used tomeasure the intensity of the flow pulse directly or indirectly. If, forexample, the increase in the internal pressure in the pressure chamberis measured using a pressure sensor, the intensity of the flow pulse canbe deduced directly from this pressure profile if the diameter of thenozzle orifice is known. This measured value can then likewise be passedto the analysis device and be taken into account there in calculationsof the other measured parameters.

In order to allow the corneal deformation to be measured as accuratelyas possible, the ray path of the slit projector should run coaxially tothe longitudinal axis of the flow pulse when impinging upon the cornea.

This can be achieved in particular by the ray path of the slit projectorrunning through the actuating device.

At the points of passage of the ray path through the actuating device,either recesses such as one formed in particular by the nozzle orificeor transparent materials should be provided, through which the lightfrom the slit projector can pass.

Alternatively thereto, deflecting optics can also be provided wherebythe ray path of the slit projector is guided past the actuating deviceand the nozzle orifice.

Which type of recording device is used to record the split images isfundamentally arbitrary. It is especially advantageous if this is ahigh-speed recording device with which split images can be recorded in arapid image sequence so that a plurality of split images can be recordedduring the deformation of the cornea. By analyzing this image sequencewhich shows the cornea from the beginning of deformation via the pointof maximum deformation until the end of deformation, the desiredmeasurement parameters can be derived very exactly.

It is particularly preferable if a video sensor is provided as recordingdevice which reproduces the image data of the split images in the formof a video signal. The video signal should preferably be produced indigital form or be converted from an analog format into a digital formatsince digital video signals in known data processing systems can beanalyzed very well by known image processing systems.

In particular, CCD chips or CMOS chips can be used as video sensorssince these make it possible to achieve high-resolution recordings andat the same time are available cost-effectively. The split images of thecornea according to the invention need not necessarily involve frameswherein the recorded section of the cornea is entirely shownpictorially. Rather, line scan cameras or a plurality of adjacentlyarranged line scan cameras can also be used as video sensors. The linescan cameras should be arranged in the recording plane such that theimage of the cornea is imaged on the line scan camera such that anydeformation of the cornea can be identified by the line scan camera.

Alternatively to using line scan cameras, area scan cameras, for examplearea scan CCD chips or CMOS chips can naturally also be used as videosensors.

In order to simplify the alignment of the eye to be examined on theanalysis system for the treating physician, the analysis system can havean adjusting camera.

BRIEF DESCRIPTION OF THE DRAWINGS

One embodiment of the invention is shown schematically in the drawingsand is explained as an example hereinafter.

In the drawings:

FIG. 1: shows the structure of an analysis system according to theinvention for measuring the intraocular pressure;

FIG. 2: shows the split image plane of an eye with nondeformed corneashown schematically;

FIG. 3: shows the split image plane of the eye from FIG. 2 with deformedcornea and arrangement of the image area of an area scan camera;

FIG. 4: shows the split image plane from FIG. 2 with deformed cornea andarrangement of the image area comprising several line scan cameras.

DETAILED DESCRIPTION

The analysis system 01 shown schematically in FIG. 1 is used to examinean eye 02 which is shown schematically. In particular, the intraocularpressure of the eye 02 and the thickness of the cornea 03 can bemeasured using the analysis system 01.

In order to be able to determine the intraocular pressure of the eye 02,the cornea 03 must be slightly deformed. An actuating device 04 is usedfor this purpose. The actuating device 04 comprises a closed pressurechamber 05 which has a nozzle orifice 06 on its side pointing towardsthe eye 02. A piston 07 which can be moved up and down by driving adrive shaft 08 in the pressure chamber 05 is used to increase theinternal pressure in the pressure chamber 05. If the piston 07 is movedinto the interior of the pressure chamber 05 with a rapid adjustingmovement, the air in the pressure chamber 05 is expelled outwardsthrough the nozzle orifice 06 and thereby produces a flow pulse whichimpinges on the cornea 03 and deforms this without contact. In order tobe able to measure the intensity of the pressure pulse, a pressuresensor 09 is provided at the pressure chamber 05 which can measure theincrease in the internal pressure in the pressure chamber 05. Theintensity of the flow pulse impinging on the cornea 03 through thenozzle orifice 06 can be derived from these pressure values.

An adjusting camera 10 is used to be able to align the eye 02 in theexact position with respect to the analysis device 01. The adjustingcamera 10 can be used to aim at the eye 02 through a transparent cover11 of the pressure chamber 05 and through the nozzle orifice 06 so thatthe treating physician can assess the correct alignment position of theeye 02 with respect to the analysis device 01. In order to be able tofix the eye 02 in the desired position during the examination, a fixinglight 12 is provided whose visible light is guided by means of themirrors 13, 14 and 15 onto the eye 02 by the actuating device 04. Themirrors 13 and 15 are constructed as semitransparent.

Further provided in the analysis system 01 is a slit projector 16 whichcan be used to project a light slit onto the cornea 03. In the slitprojector 16 the light is produced by means of a lamp 17 and is formedinto a light slit by means of a collimating slit 18. In this case, xenonhigh-pressure lamps or suitable light-emitting diodes can be used aslamps in the slit projector 16.

The light slit produced in the slit projector 16 is used to illuminatethe eye 02 in a plane of intersection which runs along the principaloptic axis 19 perpendicular to the image plane of FIG. 1. Split imagesof this type are shown schematically in FIG. 2 to FIG. 4.

A recording device 20 which can be used to observe the planes of,intersection illuminated by the light slit at an angle is used forobservation and recording before, during and after the deformation. Anobjective 21 is positioned between the recording device 20 and the eye02 such that Scheimpflug recordings can be made of the planes ofintersection of the cornea illuminated by the light beam. In therecording device 20 a CCD chip or CMOS chip is used as the video sensorwhose image data is transferred to an analysis device 24 which isinstalled as software on an industry standard PC.

FIGS. 2 to 3 are schematic diagrams showing the eye 02 in the splitimage plane illuminated by the slit projector 16. FIG. 2 shows the eye02 with undeformed cornea 03. The thickness of the cornea 03 can bededuced by image data processing of the images recorded using therecording device 20 and taken into account in the calculation of theintraocular pressure.

FIG. 3 shows the area bordered by a dashed line which can be recordedusing the CCD or CMOS video sensor built into the recording device 20.The image zone comprises a rectangular area which comprises the cornea03 at its center. During the deformation of the cornea 03, a pluralityof split images are recorded using the recording device and then theintraocular pressure is deduced by image data processing from the imagesequence of the deformed cornea 03 taking into account the thickness ofthe cornea 03 and the measurement data of the pressure sensor 09.

Alternatively to an area scan camera, one or a plurality of line scancameras can be used in the recording device 20. The image zone 23 ofthese line scan cameras is shown schematically in FIG. 4.

1. An ophthalmic analysis system (01) for measuring the intraocularpressure in an eye (02) comprising: a) an actuating device (04) forcontact-free deformation of the cornea (03), b) an observation system(20, 21) with which the deformation of the cornea can be observed andrecorded, c) an analysis device (24) with which the intraocular pressurecan be deduced from the image information of the observation system (20,21), characterised in that split images of at least parts of theundeformed and/or deformed cornea (03) can be recorded using theobservation system (20, 21).
 2. The analysis system according to claim1, wherein the thickness of the cornea (03) is deduced in the analysisdevice (24) from the split images of the cornea (03).
 3. The analysissystem according to claim 1, wherein the curvature of the cornea (03) isdeduced in the analysis device (24) from the split images of the cornea(03).
 4. The analysis system according to claim 1, wherein the lightscattering of the cornea (03) is deduced in the analysis device (24)from the split images of the cornea (03) as a measure for the elasticityof the cornea (03).
 5. The analysis system according to claims 2,wherein the thickness of the cornea (03) and/or the curvature of thecornea (03) and/or the elasticity of the cornea (03) deduced from thelight scattering is taken into account as an influential factor in thederivation of the intraocular pressure.
 6. The analysis system accordingto claims 1, wherein the intraocular pressure in the eye (02) is derivedin the analysis device (24) from the split images of the deformed cornea(03), especially from a series of split images of the cornea (03). 7.The analysis system according to claims 1, wherein the observationsystem (20, 21) cooperates with a slit projector (16) which can be usedto project a light slit onto the cornea (03) wherein the split images tobe recorded with the observation system (20, 21) lie in an image planeilluminated by the slit projector.
 8. The analysis system according toclaim 7, wherein the observation system (20, 21) comprises a recordingdevice (20) with which the cornea (03) can be recorded at least partlyin the image plane illuminated by the slit projector (16).
 9. Theanalysis system according to claim 8, wherein at least one objective(21) is arranged between the cornea (03) and recording device (20) withwhich the image plane of the cornea (03) illuminated by the slitprojector (16) is imaged on a recording plane in the recording device(20).
 10. The analysis system according to claim 9, wherein the imageplane of the cornea (03) illuminated by the slit projector (16) and theobjective plane of the objective (21) arranged between the cornea (03)and the recording device (20) and the recording plane in the recordingdevice (2) are arranged at an angle such that the image plane of thecornea (03) is imaged according to the Scheimpflug condition on therecording plane of the recording device (20).
 11. The analysis systemaccording to claims 1, wherein a flow pulse of a gaseous medium,especially air can be applied to the surface of the cornea using theactuating device (04) to deform the cornea (03).
 12. The analysis systemaccording to claim 11, wherein an at least partially transparentpressure chamber (05) comprising a nozzle orifice, (06) directed ontothe eye to be examined is provided at the actuating device (04), whereina flow pulse directed onto the eye can be produced by increasing thepressure in the pressure chamber (05).
 13. The analysis system accordingto claim 12, wherein a sensor (09) for direct or indirect measurement ofthe intensity of the flow pulse is provided in or at the pressurechamber (05).
 14. The analysis system according to claim 11, wherein theray path of the light beam produced by the slit projector (16) runscoaxially to the longitudinal axis of the flow pulse of the gaseousmedium when it impinges on the cornea (03).
 15. The analysis systemaccording to claim 11, wherein the ray path of the light beam producedby the slit projector (16) runs through the actuating device (04)wherein the actuating device (04) has recesses (06) at the points ofpassage of the ray path or is made of transparent material (11).
 16. Theanalysis system according to claim 15, wherein the ray path of the lightbeam produced by the slit projector (16) runs through the nozzle orifice(06).
 17. The analysis system according to claim 11, wherein deflectingoptics are arranged before and/or after the nozzle orifice in thepressure chamber whereby the ray path of the slit projector can beguided past the nozzle orifice.
 18. The analysis system according toclaim 1, wherein the recording device (20) is constructed in the fashionof a high-speed recording device whereby a plurality of split images canbe recorded as a series of images during the deformation of the cornea(03).
 19. The analysis system according to claim 1, wherein a videosensor is provided in the recording device (20) whereby the deformationof the cornea (03) can be observed and recorded, wherein the videosensor reproduces the corresponding image data in the form of a videosignal.
 20. The analysis system according to claim 19, wherein the videosignal is produced in a digital form or converted thereto.
 21. Theanalysis system according to claim 20, wherein the video sensor isconstructed in the fashion of a CCD chip or CMOS chip.
 22. The analysissystem according to claim 19, wherein the video sensor is formed by atleast one line scan camera.
 23. The analysis system according to claim22, wherein the video sensor is formed by a plurality of line scancameras arranged parallel to one another and at a distance from oneanother.
 24. The analysis system according to claim 19, wherein thevideo camera is formed by an area scan camera.
 25. The analysis systemaccording to claim 1, wherein an adjusting camera (10) for correctpositional alignment of the eye (02) to be examined is provided in theanalysis system (01).